1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic scheme, an electrostatic recording scheme, or the like.
2. Description of the Related Art
In general, an image forming apparatus forms visible images (toner images) by applying a developing material (e.g., toner) to electrostatic latent images formed on photosensitive drums, and primarily transfers the formed visible images to an intermediate transfer material. The image forming apparatus then secondarily transfers the primarily transferred toner images altogether onto a recording material.
In an image forming apparatus of this type, a “density shift” may occur for each color due to variations in transfer characteristic based on a toner residual amount and/or ambient temperature. In an image forming apparatus using a plurality of photosensitive drums, so-called “color misalignment” may occur. “Color misalignment” occurs when the image formation positions of toner images of the respective colors disagree due to the mechanical mounting errors between the respective photosensitive drums, errors in the optical path length between laser beams, and/or changes in optical path.
In general, automatic adjustment for “density shift” is executed by the following procedure. First of all, reference patterns using toner images are formed on a photosensitive drum or an intermediate transfer belt. Photosensors detect the densities of the formed toner images. Process conditions or correction values for Υ (gamma) characteristics are automatically controlled to make the detection results have predetermined values. This makes it possible to stabilize the image density.
On the other hand, automatic adjustment for “color misalignment” is executed by the following procedure. First of all, photosensors read reference patterns of the respective colors which are formed on an intermediate transfer material by the respective image forming parts. The photosensors are placed adjacent to a photosensitive drum located on the most downstream position. The positions of reference patterns of the respective colors on the intermediate transfer material are detected on the basis of the read results. Automatic adjustment of the output timings of image signals to be recorded or image signals themselves is electrically performed to set predetermined intervals between the reference patterns of the respective colors. It suffices to automatically adjust an optical path length by driving a return mirror provided in the optical path of a laser beam.
The automatic adjustment disclosed in Japanese Patent No. 3450402 normally demands maximum density correction and gray level correction for each color. In addition, in automatic adjustment of color misalignment, it is necessary to form reference patterns several times so as to reduce errors such as the decentering of a driving system. Therefore, it may take several minutes to execute automatic adjustment.
In addition, in consecutively forming images on a plurality of sheets, the apparatus may execute these automatic adjustment operations between the end of image formation on a given sheet and the start of image formation on the next sheet (a so-called sheet-to-sheet interval). In this case, the apparatus cannot perform image formation until the completion of automatic adjustment. This leads to a reduction in productivity and long waiting times.
Japanese Patent Application Laid-Open No. 2002-91096 discloses an automatic adjustment method of adjusting the number of reference patterns to be formed on a sheet-to-sheet interval secured on an intermediate transfer belt.
However, with an increase in the speed of image forming apparatuses and an improvement in productivity, the sheet-to-sheet interval has become shorter than that in conventional apparatuses. This makes it difficult to form a sufficient number of reference patterns on the sheet-to-sheet interval. If the sheet-to-sheet interval increases for automatic adjustment, the down time undesirably increases. It is therefore preferable to minimize the execution of automatic adjustment which requires an increase in sheet-to-sheet interval.
In addition, the above conventional technique gives no consideration to the type of post processing to be executed for a recording material on which an image is formed. However, there is a possibility that automatic adjustment processing cannot be executed depending on the type of post processing to be performed.